flatten 2 ” slope on roof to pop up
anyriver
| Posted in Construction Techniques on
I am designing a second floor to go over my flat roofed garage that has been remodeled into an office. The problem is that the flat roof is not flat but slopes 2 ” across the 20 ft width. I also have to raise the floor 4 inches to match the rest of the second floor. I would like to minimize the damage to the ceiling of the office underneath. I need clever ideas. Incidently, the foundation will support the extra floor and I live in moderate snow country.
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How deep are the existing ceiling/roof joists of the garage/office? Seems like all you would have to do is remove the roof deck and lay in new floor joists that are tall enough to raise the floor level to what you need. In addition, you will probably get deep enough fllor joists to soan that far and support the floor loads. Chances are that the existing sloped roof joists are not strong enough to support a floor.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
Two inches in 20 feet? Heck, our kitchen floor slopes more than that!
Yeah, good thing you are building the seond floor, there is not enough slope for a roof.
To clarify tho, are you building ON the roof or UP from the walls? What size, span and layout aare the existing joists?
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The joists are 2x12 on 16" centers going across the space. I am building up from the 2X4 walls. Thanks for your feedback
across the space?Does that mean they are spanning 20'?
If so, you should plan to sister to double them up for a floor
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If he needs to raise the floor 4", why not sister in 16" I-joists?
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
I never consider I-joists for sistering, but your point about the depth is well made. As was pointed out, an engineer can do this with cost analysis in mind
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Thanks for the feedback. I have an engineer who will be out next week to look at the project and give his ideas. The span is 20'. The only problem with sistering is that it currently is a well insulated ceiling with wiring running through it and copper pipes for a solar hot water system which is on the adjoining roof. It will be a lot more work to get into the roof if the build up needs to go all the way down to the inside sheetrock. A 2x8 that is isstered could probably be acomadated without much extra effort. I will look into the RC clips.
Good thing you have an engineer coming then, because IMO, no way will that be enough for a floor spanning 20'
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I am wondering if you can build an additional floor deck with joists that are perpendicular to the original joists? Just lay the new "box" ontop?
You won't be disturbing any wiring/piping etc in the original joist cavities.
I guess it all depends upon whether the existing joists can handle all the new load.
You could also "float" a new floor deck on top of the existing joists. Properly sized I-joists which are supported by the existing walls below. Or are supported by columns that are placed within the existing walls below at the corners. The new joists do not touch the existing joists.
The new floor would most likley be a bit higher than the existing 2nd floor. But that might be an interesting feature that you step up into the new space.
That would be adding load without adding strength and pretty much gaurantee a sagging floor system
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Are you refering to my first idea about a new floor box laid perpendicular to the existing joists? That's why I also said, "I guess it all depends upon whether the existing joists can handle all the new load."
Right. The existing joists definitely cannot be expected to perform as floor joists at that layout and span
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The existing joists definitely cannot be expected to perform as floor joists at that layout and span
For most common materials, it meets code and is fine - 2x12's 16" OC SYP code at 20'-9", hem-fir at 19'-11" - both are plenty for that span. No reason to think the structure is not adequate for what the poster proposes. For other than residential bedroom or office use, though, I'd be more comfortable with a little more structure.
Still, it's a good idea to have an engineer look at it - I'm not going to risk MY license giving advice on the web :-)
OK, it does depend on species and grade. I was going by what we commonly get around here which would allow 15'10" up to about 17'. The best I can get from tables other than select structural DF(21'1") is 19'7"We don't know what he has tho so both our assumptions are out there floating around. Neither do we know the specifics on all these holes for pipes and wires and whether they were properly drilled.So it is right back to the engineer on site to ascertain what he needs to do.
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A 2x8 that is isstered could probably be acomadated without much extra effort.
I wonder if you are using the right term. If you sister a new 2x8 to the existing joist, that means that you would lay them side by side, both resting on the same top plate. If you notch the new 2x8 to fit around pipes, then you reduce it's effective size to a 2x4.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
But many of them would be four inches higher than the tops of existing so he would not need any notching in those...Still, he is going to need to do more than he seems to be expecting to gain what he wants.
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Ok, I'm confoosed. He talks about sistering joists, and in response tomy suggestion of using tall I-joists he says that he has pipes and wires in the way. Let's say that the pipes are resting on top of the old joists, and the old joists are 2x8, and the new I-joists are 14" high. So thenm he has to cut an 8" notch out of the bottom of a 14" joist at near mid-span, so then he is left with a 6" joist. Or am I missing something?
If he adds a new joist on top of the old, then he's not sistering the joists.
Sounds like he's doing it my way: instread of clearing the way and going back correctly and easily, he's trying to rube-goldberg a solution.
"When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it." T. Roosevelt
You have totaally lost me with thaat scenario. He has 2x12 now, not 2x8He is talking of adding 2x8, not that that would help him much.
Neighther of us can see the bearing plate conditions, but a sister can be blocked or shimmed up - it does not need to rest on the plate directly
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Thanks for the comments. I was using the term sister incorrectly. The idea was to put some type of 20 " wood next to each 2 x 12 but only going a max of 6 inches down the side of the 2 x 12 's It could be blocked and/or atached to the rim joists and could rest on the blocks which rest on the top plate. which I think is doubled. A versalam is probably better than an I joist since it could be both glued and nailed or screwed to the current 2 x 12 ''s. Will I get additional strength from the modified sistering or will the versalam have to be good for the whole span by itself? If the versalam has to be stong enough by itself it would have to be 20' by 3 x 9 1/2 high and cost about $2 50 a linear ft.
Only your engineer knows for sure. I do my simple engineering but when it gets off th escale like this with unique situations, I always take it to him for confirmation.
My tendency is to think that there can be a way to combine the LVLs with what you have, but it may be very hard to prove on paper. The existing will be doing the work in the tension area while the lvls will do more of the work in the compression area, so you will likely still need to remove the ceiling too and do something on that tension plane
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You said that you have to raise the floor up 4" anyway. Why can't you install 3-1/2" x 9-1/2" I-joists? They can span 20'. That will give you about 5-3/4" from the bottom of the I-joist to the bottom of the 2x12 if the 2x12 is about 11-1/4".Will you be able to sit both ends on top plates?Joe Carola
Edited 10/31/2006 8:12 pm ET by Framer
I have an engineer who will be out next week to look at the project and give his ideas. The span is 20'. The only problem with sistering is that it currently is a well insulated ceiling with wiring running through it and copper pipes for a solar hot water system which is on the adjoining roof.
I suggest you ask your engineer to design you a railroad truss which can be built on-site so you can work around the piping, wiring, etc., that you don't want to disturb. Like Piffin, I don't find any solid dimensional lumber in my tables which will span 20 feet for a floor, and you certainly don't want to start boring or notching I-joists or LVL's or other faith-based structural members....
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How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
"faith-based structural members.."Great phrase - mind if I borrow it?Good idea too
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Sure go ahead. And thanks for the compliment on the idea. Just a little something I found lying around outside the box....Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
Like Piffin, I don't find any solid dimensional lumber in my tables which will span 20 feet for a floor,
What tables are you using? Here's one from the SYP web site.
Maybe the problem is the load rating you are using? 40psf live and 10psf dead is BOCA - are you using something else? For those loads, 2x12 @16OC are fine (as the attached span table shows).
I'm using the NBCC tables, which generally correspond fairly closely to those published in the US although we do have more extensive environmental data (like snow load) incorporated in things like rafter tables (for obvious reasons).
The best I could come up with for "Floor Joists, General Cases," would be using Structural Select DF/Larch or Hem/Fir. Even at 12"oc 2x12 with bridging and strapping, the maximum span is 5.66 meters (18.57 feet). If we go down to 16" oc (what this poster's actually got now for his ceiling joists) the span would shrink to 5.28m (17.32') for the same lumber & conditions.
If I use the "Special Cases" table, I can come up with 6.37m for 2x12 16oc with concrete topping. (Without the topping but with bridging and a ceiling attached to strapping, the max span is 5.89m.)
20 feet is a very big span for residential construction. Not many rooms in a normal house need to be that big, especially with a clear span.
I think a railroad truss designed for his application is going to be the best way to solve all his problems simultaneously.
Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
The best I could come up with for "Floor Joists, General Cases," would be using Structural Select DF/Larch or Hem/Fir. Even at 12"oc 2x12 with bridging and strapping, the maximum span is 5.66 meters (18.57 feet). If we go down to 16" oc (what this poster's actually got now for his ceiling joists) the span would shrink to 5.28m (17.32') for the same lumber & conditions.
You raise good points. The species and grade affect the span, as well as which standard is applied. For example, I was using the BOCA L/360 requirement, but as I think about it, for a span that long the floor is likely to be a little "springy" to satisfy most customers.
It sounds like your span tables incorporate strapping and bridging? Usually US span tables are just straight spans.
I think a railroad truss designed for his application is going to be the best way to solve all his problems simultaneously.
Wasn't disagreeing with your suggestion (which makes a lot of sense for this application), just discussing allowable spans.
It sounds like your span tables incorporate strapping and bridging?
They have three columns: with strapping; with bridging; with both.
Just to give you an idea, SPF #1&2 2x12's @ 16o.c. yield 4.52m (bridging), 4.71m (strapping), or 4.81m (both).Dinosaur
How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....
U need to be able to buy SYP framing lumber before using a table seet up for it.I use the span calc download from the canadian wood council
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U need to be able to buy SYP framing lumber before using a table seet up for it.
True. I chose SYP because it is readily available in most parts of the US - seemed like it would be a "lowest common denominator". Wasn't thinking about folks in Canada and elsewhere. Sorry.
Without knowing the specifics of your situation... check into a Simpson item called the "RC" or "Ripper Clip". It is designed to allow a piece of 2x lumber to piggyback on another piece of 2x lumber. The usual application is to allow a tapered rip to sit on top of a ceiling joist, creating a sloped roof (which you already have). The opposite would probably also work, meaning you could rip a taper that when applied to your roof framing made it level.
Since no one else has taken the opportunity yet, I'll say it... you should get a structural engineer to design your addition for you. It probably won't cost very much in the overall scheme.